1. Technical Field
This invention relates generally to a method for controlling hydrocarbon emissions from a gasoline, natural gas, or propane fueled engine, and more particularly to such a method for reducing hydrocarbon emissions during an initial engine operating period following a cold-start.
2. Background Art
Catalytic converters are commonly used to convert environmentally harmful tail pipe emissions from automotive internal combustion engines, such as hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NO.sub.x), to less harmful products such as carbon dioxide (CO.sub.2), water (H.sub.2 O), and nitrogen (N.sub.2). However, acceptable vehicle emission levels are becoming increasingly lower as evidenced by regulations imposed by the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) in the U.S.A., and by other government bodies in Europe and other areas of the world. Recent CARB standards, such as the Low Emission Vehicle (LEV) and the Ultra Low Emission Vehicle (ULEV), require extremely effective catalytic converters.
One of the main challenges to achieving lower overall emissions has been the problem of cold-start HC emissions. An engine cold-start is defined in 40 CFR .sctn.86, Subpart B, Appendix 1, titled Urban Dynamometer Driving Schedule for Light Duty Vehicles and Light Duty Trucks. Typically, the catalyst material in a catalytic converter must reach a sufficiently high temperature before it is capable of converting the harmful emissions. Therefore, there is an initial period of engine operation from a cold start before the catalytic material is heated to its active conversion temperature. This initial period typically extends from cold-start for about 45 seconds to as much as 240 seconds or longer, depending on catalyst volume and converter-exhaust system design. In addition to the catalytic material having a sufficiently high temperature, the exhaust gas must also be stoichiometric or lean, i.e. have a .lambda. value that is equal to or greater than 1, before the catalyst can effectively convert HC to CO.sub.2 and H.sub.2 O. During cold-start, the exhaust gas mixtures are often fuel-rich (.lambda.&lt;1), to aid in efficient engine starting. The initial fuel-rich exhaust gas mixture, combined with low catalytic material temperature, result in a significant amount of the overall HC emissions being generated during the initial engine operating period following a cold-start. Once the engine reaches a designated operating temperature and comes under stoichiometric control, the HC emissions generally can be effectively reduced using current technology.
Reduction of cold-start HC emissions has been an area of considerable research and development in recent years. Several technologies have been developed in an attempt to reduce cold-start HC emissions. For example, secondary air, using air pumps, can be added to the exhaust stream to make the cold-start exhaust gas stoichiometric or lean at the catalyst, alleviating the problem of rich engine-out exhaust. However, this option adds weight, costs, and complexity to a vehicle, and for those reasons is not a desirable option. The secondary air is also relatively cool which can slow down the light-off of the catalyst.
Other attempts to overcome the discharge of relatively high levels of HC during the cold-start period of an engine include the use of Electrically-Heated Catalysts (EHCs). EHCs induce rapid catalyst heatup and accordingly faster light-off, i.e. the temperature at which the catalytic material becomes effective in promoting a catalytic reaction with the exhaust gas. EHCs can be very effective, but they also require considerable energy, which reduces the overall fuel efficiency of the vehicle. Stoichiometric, or lean engine-start strategies can also be used, but such strategies can cause problems with operating smoothness of the engine during the cold-start period. Latent heat devices have been demonstrated which maintain the catalyst at or near its operating temperature for several hours after the engine is switched off, allowing for faster light-off when the engine is restarted. Also, hydrogen has been pumped into the exhaust gas stream at a point before entry of the gas stream into the catalytic converter to facilitate faster catalyst light-off. Latent heat devices and hydrogen supplement systems add significant hardware, controls, complexity, and cost to an engine.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a method for operating a gasoline, natural gas, or propane fueled, spark-ignition engine during a cold-start, without introducing excessive hydrocarbons into the surrounding environment. It is also desirable to have such a method that does not require lean-start control strategies, electrically-heated catalysts, latent heat devices, secondary air, or hydrogen injection. It is also desirable to have such a method that does not require extensive additional hardware or engine operating controls in order to function effectively.